1. Field of the Invention
The present invention pertains to squeegees used in screen printing and in particular to squeegee assemblies and the stiffness of the squeegee blade used therein.
2. Description of the Prior Art
In many known types of manually and automatically operated screen printing apparatus, a squeegee is used to distribute ink across the screen and to force the ink through the screen onto the workpiece to be printed upon in a pattern determined by art work formed on the screen.
Improved printing characteristics have been obtained by some screen printers by stiffening the squeegee with a metal support on the back side of the squeegee. The tendency of the squeegee rubber to be deflected and/or bent during the print stroke was decreased by the stiffness added by this metal plate behind the squeegee. By reducing the tendency of the squeegee rubber to bend or deflect away, the sharp edge of the squeegee is presented to the screen to "cut off" the ink more cleanly at the screen mesh line, thereby providing a more uniform squeegee function. One proposal is to adhere a rigid plastic backing to the squeegee blade to increase its stiffness.
Much of the screen printing today is done with automatic or semiautomatic screen printing equipment which may be used to print with various kinds of inks, through various kinds of screens, and onto various kinds of substrates. Thus, a squeegee in such equipment is subjected to varying printing conditions and there still is a need for a sharp edge to be presented to the screen to cut off the ink cleanly at the screen mesh line. Turning first to inks, many different types of printing inks are used in screen printing today and some of the inks, being more viscous than others, require a greater pressure on the squeegee to force a desired amount of ink through the screen. Also, temperature and humidity conditions may have some effect on the printing pressure, as will screen fabrics.
Also, the amount of ink applied to a workpiece varies with the surface characteristics of the workpiece material. Today, screen printing is used on a variety of workpiece materials, such as paper cardboard, but also cloth, metal, glass and ceramics. These may require widely varying printing pressures to assure printing patterns having the desired sharpness and detail. Also, there is the requirement of blade sharpening which is usually done with the clamp in position on the blade. Hence, devices which control the blade stiffness should not interfere with the blade sharpening operation.